Radiation resistant fiber optical assembly

ABSTRACT

An optical amplifier is provided in which radiation levels experienced by an optical fiber are minimized. A sealed enclosure houses an optical fiber. An input optical signal enters on end of the fiber and an amplified output optical signal exits the other end of the optical fiber. Small particles embedded with a gas fill the interior of the enclosure. An optical pump supplies an amplification laser beam coupled to one of the ends of the optical fiber. Due to energy supplied by the amplification laser beam, the input optical signal is amplified upon exiting the optical fiber. At least a portion of the gas embedded in the small particles is released inside the enclosure when the small particles are subjected to heat to provide a gaseous interior in the enclosure that minimizes radiation levels experienced by the optical fiber due to external radiation.

BACKGROUND

This invention relates to fiber optic amplifiers that must operate inenvironments subject to exposure to significant radiation.

Fiber optic amplifiers have been used in various applications, e.g.telecommunication in trans-ocean optical fibers. The principle of signalamplification is based on solid state quantum effects where a high powerpump laser beam causes electron population inversion in the fiber energybands and the stimulated photon emission amplifies the incoming opticalsignal. To match the energy levels in the optical fiber with pump laser,optical fibers may be doped with rare earth materials, e.g. erbium,ytterbium, etc. The doped rare earth elements generate multiple newenergy bands in the glass fiber. The photonic energy from a shortwavelength pump laser excites charge carriers and leads to populationinversion in the energy levels corresponding to the short wavelengthphotons. Upon stimulation by an incoming laser that carries datapackets, the charge carriers undergo an emission of photons giving riseto amplification of the incoming laser carrying the data packets.

Fibers doped with rare earth elements darken when subjected tosignificant radiation levels, e.g. outer space, causing degradation ofoptical amplification and an increase in optical amplifier's noisefigure. Although attempts to produce special fibers developed to bettertolerate radiation have been made, these special fibers tend to losetheir amplification efficiency. There exists a need for an improved wayto reduce degradation of rare earth-doped optical fiber due to radiationexposure.

SUMMARY

It is an object of the present invention to satisfy this need.

An exemplary optical assembly and optical amplifier using the assemblyare provided in which radiation levels experienced by an optical fiberare minimized. A sealed enclosure houses an optical fiber. An inputoptical signal enters one end of the fiber and an amplified outputoptical signal exits the other end of the optical fiber. Small particlesembedded with a gas fill the interior of the enclosure. An optical pumpsupplies amplification laser beam coupled to one of the ends of theoptical fiber. Due to energy supplied by the amplification laser beam,the input optical signal is amplified upon exiting the optical fiber. Atleast a portion of the gas embedded in the small particles is releasedinside the enclosure when the small particles are subjected to heat toprovide a gaseous interior in the enclosure that minimizes radiationlevels experienced by the optical fiber due to external radiation.

An exemplary method minimizes radiation levels experienced by an opticalfiber that must operate in an environment containing radiation. Themethod includes disposing a length of optical fiber in an enclosurewhere the optical fiber has first and second ends; the enclosure alsoenables the coupling of input and output optical signals to therespective first and second ends of the length of optical fiber. Gas isinfused under high pressure into small particles coated with a resilientmaterial that is permeable to the gas. The interior of the enclosure notoccupied by the optical fiber is filled with the gas infused particlesand the enclosure is sealed. Heat is controllably supplied to heat thesmall particles so that at least a portion of the gas embedded in thesmall particles is released inside the enclosure to provide a gaseousinterior environment in the enclosure that minimizes radiation levelsexperienced by the optical fiber due to external radiation.

DESCRIPTION OF THE DRAWINGS

Features of exemplary implementations of the invention will becomeapparent from the description, the claims, and the accompanying drawingsin which:

FIG. 1 is a block diagram of an exemplary optical fiber amplifier inaccordance with an embodiment of the present invention.

FIG. 2 illustrates an enclosure that holds a coiled length of rare-earthdoped optical fiber and protective particles in accordance with anembodiment of the present invention.

FIG. 3 is a representative cross-sectional view of the enclosure showingthe coiled optical fiber surrounded by the protective particles.

DETAILED DESCRIPTION

One aspect of the present invention resides in the recognition of thedifficulties associated with the use of a gas, e.g. hydrogen ordeuterium, which fills an enclosure to provide protection for anenclosed optical fiber against radiation due to the optical fiber beingsurrounded by the gas which reduces the darkening of the fiber. Thistypically requires the use of a hydrogen storage tank coupled to theenclosure. The tank fills and maintains a level of gaseous hydrogenaround the fiber to account for leakage and loss of some of the hydrogenover time. However, requiring a hydrogen storage tank is a substantialburden in weight, reliability and safety. These burdens are especiallytroublesome where the optical fiber is part of a payload to be launchedinto space for operation and the likelihood of leakage of the gas fromthe enclosure is high given a very low pressure external environment,i.e. the vacuum of space.

FIG. 1 shows a block diagram of an exemplary optical fiber amplifier 10in accordance with an embodiment of the present invention in whichprotection against the effects of radiation is provided withoutrequiring an external gas supply. An optical fiber assembly 12 includesan enclosure 14, rare-earth doped optical fiber, e.g. an optical fiberdoped with erbium, ytterbium, etc., wound into a coil 16, and aplurality of particles 18 that substantially fills the space withinenclosure 14 not occupied by the optical fiber. In this illustrativeexample, the rare-earth used is erbium. The enclosure 12 includes anentry port 20 and an exit port 22 which provides a hermetic seal whileallowing the optical fiber to enter and exit the enclosure. Theenclosure 12 may be made of an austenitic stainless steel. The endplates (entry and exit ports) may be made of a metallic material withlow coefficient of thermal expansion, e.g., an iron-nickel alloy, inorder to match with a glass seal of hermetic connectors. The end platescan be either fusion welded or brazed to the austenitic stainless steelenclosure. An optical input source 24, e.g. a laser modulated with data,provides a signal 25 that is coupled as an input to a coupler 26.Another port 30 of coupler 26 carries the signal 25 that is advancingtowards assembly 12 and the optical energy provided by optical pump 29.Port 28 of the coupler 26 carries the optical energy generated by theoptical pump 29.

The coupler 34 includes a port 32 which carries signal 25 moving awayfrom the enclosure 14 after being amplified primarily within enclosure14 and the energy from optical pump 29 that is advancing towards theenclosure 14. Port 36 of coupler 34 receives the optical energy fromoptical pump 29 which supplies energy providing for the application ofsignal 25. Port 38 of coupler 34 carries the amplified signal 25 whichis provided as an input to the optical isolator and filter 40 which inturn provides an output signal that is provided as an input to powermonitor 42 before being coupled as a final amplified optical outputsignal 44. A control signal 46 from the power monitor 42 correspondingto the level of the amplified signal 44 is provided to the heatcontroller 48 which controls the amount of current that flows throughheating element 50 that is, in this example, attached to the outside ofthe enclosure 14.

The principle of signal amplification is based on solid state quantumeffects where a high power pump laser beam causes electron populationinversion in the fiber energy bands and the stimulated photon emissionamplifies the incoming optical signal. To match the energy levels in theoptical fiber with pump laser, rare-earth doped optical fibers are used.The doped rare-earth elements generate multiple new energy bands in theglass fiber. The photonic energy from a short wavelength pump laserexcites charge carriers and leads to population inversion in the energylevels corresponding to the short wavelength photons. Upon stimulationby an incoming laser signal that carries data packets, the chargecarriers undergo a stimulated emission of photons and gives rise toamplification of the incoming laser signal.

The particles 18 may be powders of palladium with sufficiently smallsize, e.g. 50 to 200 micrometers, that are preferably coated with athin, e.g. 10 to 20 micrometers, gas permeable coating, e.g. silicone orurethane coating. The coated powders are charged at high pressure, e.g.10 to 100 atmospheres, with a suitable gas that can provide radiationprotection, e.g. hydrogen or deuterium, at a temperature 25 C-200 C.Assuming that palladium is used, it can absorb approximately 2700 timesthe amount of hydrogen of its own volume. Hence, there is more thanenough supply of hydrogen available from these particles. The protectiveeffect is believed to be chemical based. As long as there is hydrogengas presence inside the enclosure, there will be sufficient hydrogendiffused into the optical fibers to substantially prevent or cleardarkening of the fiber due to space radiation. After gas loading, thepowders are then filled into the space inside the hermetic enclosure 14not occupied by the fiber 16. The soft silicone or urethane coating onthe powders allow the powders to directly contact otherwise sensitiveerbium doped optic fiber without introducing hard points or acuteinflections to the fibers that could cause signal loss/degradation dueto micro bending. The hermetic enclosure 14 is then sealed by the endplates. The heater 50, instead of being attached on to the outsidesurface of the enclosure 14, could also be embedded in the walls of theenclosure 14 or disposed inside the enclosure as long as it does notinterfere with the optical fiber or optical amplification. In a spacemissions, the heater can be powered to allow the doped fiber to reach acontrolled temperature of 70 C to 140 C so that the embedded gas, e.g.gaseous hydrogen, is released from the particles which act as storagemedia for the gas. In such an application, the heater can be turned offduring most of the time of the mission and only turned on as requiredwhen it is detected that the level of amplification has dropped below apredetermine level because of space radiation.

FIG. 2 illustrates the exemplary enclosure 14 that holds a coiled lengthof rare-earth doped optical fiber 16 and particles 18 embedded with gasthat, when released, provides radiation protection in accordance with anembodiment of the present invention. In this embodiment the entry andexit couplers 26, 34 are internal to the enclosure 14. Even though theports 20, 22 are intended to provide a hermetic seal, it is verydifficult to prevent some amount of long term leakage of a highlypermeable gas such as hydrogen especially in the lower pressureenvironment of space.

FIG. 3 is a representative cross-sectional view of the exemplaryenclosure 14 showing the coiled optical fibers 16 surrounded by theprotective particles 18. When the protective particles 18 are heated asexplained above, the gas embedded in the particles escapes through thegas permeable coating on each particle to fill the vacant interior spaceof enclosure 14 with gas which minimizes the effects of externalradiation upon the contained fibers. The particles 18 also provide thecoil 16 of optical fiber a cushion against shock and vibration whichwould be experienced during the launch of a space vehicle carrying theoptical amplifier 10.

Although exemplary implementations of the invention have been depictedand described in detail herein, it will be apparent to those skilled inthe art that various modifications, additions, substitutions, and thelike can be made without departing from the spirit of the invention. Forexample, other hydrogen storage materials may be used for making theparticles. These materials include but are not limited to graphite,carbon nanotubes, Zeolites, metal organic frameworks, organic polymers,metal hydrides, complex hydrides, Amides, Imides and Mixtures, ClathrateHydrates, and a combination thereof. A different hydrogen or deuteriumloading procedure may be used based on the hydrogen storage materialselected. Pump lasers may be integrated into the hermetic enclosure. Assuch, the number of hermetic fiber connectors can be reduced andreplaced with hermetic multipin electrical connectors that provideelectrical power to the pump laser and the thermal electric coolerinside the pump laser modules. If the pump laser modules become part ofthe hermetic enclosure, the material of the enclosure should be selectedfrom materials with high thermal conductivity so that the enclosurebecomes a heat sink for the pump laser when properly mounted to asupporting baseplate.

The scope of the invention is defined in the following claims.

The invention claimed is:
 1. An assembly for minimizing radiation levelsexperienced by an optical fiber comprising: a sealed enclosure; a lengthof optical fiber disposed in the enclosure and having first and secondends; at least one port on the enclosure that enables the coupling ofinput and output optical signals to the respective first and second endsof the length of optical fiber; a plurality of small particlessubstantially filling the interior of the enclosure where the particleshave embedded therein a gas which, when released, reduces the effects ofradiation upon the optical fiber; a heating element disposed proximateto the small particles; a heat controller coupled to the heating elementthat controls whether heat generated by the heating element is coupledto the small particles; at least a portion of the gas embedded in thesmall particles being released to fill the inside the enclosure andprovide a gaseous interior environment totally surrounding the length ofthe optical fiber when the small particles are subjected to the heatfrom the heating element to provide a gaseous interior environment inthe enclosure that reduces the effects of radiation from all directionsupon the optical fiber due to radiation for a source external to theenclosure.
 2. The assembly of claim 1 wherein the optical fiber is dopedwith a rare-earth element.
 3. The assembly of claim 1 wherein the atleast one port provides a substantially hermetic seal of the closure. 4.The assembly of claim 1 wherein the length of optical fiber is formed asa substantially circular coil.
 5. The assembly of claim 1 wherein eachof the small particles is covered by a thin resilient coating that ispermeable to the gas.
 6. The assembly of claim 1 wherein the heatingelement abuts an exterior wall of the enclosure.
 7. The assembly ofclaim 2 wherein the rare-earth element comprises at least one of erbiumand ytterbium, and the gas comprises at least one of hydrogen anddeuterium.
 8. The assembly of claim 1 wherein the small particlescomprise palladium.
 9. The assembly of claim 2 wherein the rare-earthelement comprises at least one of erbium and ytterbium, the gascomprises at least one of hydrogen and deuterium, and the smallparticles comprise palladium.
 10. The assembly of claim 1 furthercomprising: an optical input source that supplies an input opticalsignal coupled to the first end of the optical fiber; an optical pumpthat supplies an amplification laser beam coupled to one of the firstand second ends of the optical fiber; the input optical signal beingamplified upon exiting the second end of the optical fiber due to energyprovided by the amplification laser beam which exits the other of thefirst and second ends of the optical fiber.
 11. The assembly of claim 10further comprising: a monitor that monitors the power level of theamplified optical signal, the monitor providing an output control signalcoupled to the heat controller that controls whether the heat controllercauses the heating element to generate heat.